Well, it would depend on your definition of "better." Demosthenes left a big imprint on western civilization through his oratory (and the fact that a sizable bit of his corpus survives). Almost three centuries later, Cicero admired and modeled himself after Demosthenes. That influence undoubtedly added to the high standing which Athens holds in the historical consciousness of Europe.

As for whether Athens would have been better off with or without Demosthenes in his own lifetime, that is, of course, debatable. Personally I tend to agree with Ian Worthington. In his history of Philip II, he pretty convincingly argues that Philip was willing to negotiate with Athens, even to his own detriment, up until the very last moment when conflict became inevitable. For example, at the end of the Third Sacred War Athens and her allies were in a weak position. Philip (fighting for the other side, with the backing of Delos and Thebes) would not have had much trouble squashing Athens right then and there. Instead, however, he chose to seek peace with Athens and orient himself against Thebes. Philip seemed happy enough to keep his army out of Athens (with the notable exception of some Athenian colonies/allies on his borders) until Demosthenes precipitated a widespread military alliance against him. That is not to say that Athens would have remained independent without Demosthenes there to stir the pot, but I think it is reasonable to argue that without Demosthenes' constant agitation against Philip Athens may have had a chance of remaining semi-independent under Macedonian hegemony.